Recently in the field of biotechnology, remarkable developments are being made, applications to other fields are being sought, and research on application to semiconductor microfabrication technology (bionanotechnology) is also being pursued. In the biotechnology field, products (amino acid residues) can be controlled at the molecular level based on a blueprint called “DNA,” and all proteins formed from the various amino acid residues have a self-assembling ability that enables forming of “nanoblocks” that are not dispersed in size. Thus by employing biotechnology, the formation of a product can be controlled at the nanoscale, and in the incorporation of a semiconductor component in the product, application of biotechnology to semiconductor microfabrication technology becomes apparent.
Here, as arts in which biotechnology is applied to semiconductor microfabrication technology, arts of forming a quantum dot using a cage-like protein called ferritin, which is present in living bodies, have been disclosed (refer to Patent Document 1 and Patent Document 2).
As shown in FIG. 1, ferritin has a structure that includes a spherical protein outer shell portion, which has a diameter of 12 nm and is made up of 24 protein monomer, and a core portion 1A, which is a central portion of the outer shell portion, has a diameter of approximately 6 nm, absorbs Fe ions from inside a living body, and holds the ions in the form of an oxide. Ferritin is called a cage-like protein due to having the protein outer shell portion and the core portion. Ferritin has an active site that oxidizes the absorbed Fe ions, and the Fe ions are accumulated as the oxide, 5Fe2O3.9H2O.
A protein with which the metal oxide core is removed from the ferritin is called apoferritin, and besides Fe, apoferritin is capable of accumulating microparticles made of various metals, such as nickel (Ni), cobalt (Co), manganese (Mn), etc. TEM images of various metal nanoparticles prepared inside apoferritin are shown in FIG. 2
As described above, ferritin has the structure including the metal oxide in the core portion and the outer shell in which 24 protein monomers are assembled together. The ferritin has a self-assembling ability and can thus be formed readily as a uniform film, and the outer shell proteins have a characteristic of being readily decomposed and removed by UV ozone heat treatment, etc.
Here, the self-assembling ability of ferritin can be utilized to control an adsorption position of ferritin on a semiconductor substrate, and by selectively removing the outer shell protein of the ferritin, a structure with which the core metal oxide is aligned in a two-dimensional matrix as shown in FIG. 2 can be prepared.
It is also known that, with ferritin, a nanoparticle of a compound semiconductor, formed of two or more types of elements, can be prepared in the core portion (refer to Patent Document 3).
Due to a quantum confinement effect, a nanoparticle of a compound semiconductor expresses physical properties that differ greatly from those of a bulk state. In particular, such nanoparticles have ideal fluorescence characteristics, such as a high luminance, a high light resistance, a broad excitation spectrum, and a narrow fluorescence spectrum, and are thus attracting attention as next-generation optoelectronic materials. Meanwhile, since the development of a water-soluble compound semiconductor nanoparticle that uses a hydrophilic coating molecule, active research is being carried out on applications of compound semiconductor nanoparticles to biotechnology, such as bioimaging, immunoassay, etc.
By the art disclosed in Patent Document 3, use of a quantum size effect of a microparticle made of a semiconductor has become possible, and in a case of a compound semiconductor microparticle that emits fluorescence when excited, use in a biological substance labeling method, etc., has become possible. TEM images of various compound semiconductor nanoparticles prepared inside apoferritin are shown in FIG. 3.
As described above, compound semiconductor nanoparticles are gathering attention as a next-generation optoelectronic material. However presently, the circumstances are such that the uses of compound semiconductor nanoparticles in the biotechnology field are limited to alternatives to light-emitting organic molecules. Up to now, the present inventors, with an aim at creating highly luminescent circularly polarized light-emitting molecules, have created helical polymers, aromatic low-molecular-weight molecules, and compound semiconductor nanoparticles that are optically active and light-emitting and have examined circularly polarized luminescence (CPL) characteristics of these substances.
Circularly polarized luminescence (CPL) refers to a difference in emission intensities of right circularly polarized light and left circularly polarized light emitted from an optically active molecule (see FIG. 4). Although such circularly polarized luminescence (CPL) has been used from before for evaluation of a steric structure of an organic molecule in an excited state, application to polarized light sources for high-luminance liquid crystal displays as well as three-dimensional displays, memory materials, optical communication, and other forms of advanced light information processing are being anticipated recently.
As substances exhibiting circularly polarized luminescence (CPL), bioluminescent substances, light-emitting rare earths, optically active conjugate polymers, etc., are known. Among compound semiconductors, GaAs has been reported to exhibit circularly polarized luminescence when excited by a circularly polarized laser. However, there have been no reports so far of circularly polarized luminescence being achieved with a compound semiconductor nanoparticle (see, for example, Non-Patent Document 1). Although as an attempt to see if a compound semiconductor exhibits circularly polarized luminescence (CPL) has been made in regard to circular dichroism (CD) active CdS synthesized from an optically active thiol compound, this report indicated that the compound semiconductor nanoparticle is CPL inactive (see Non-Patent Document 2).